Nitro substituted aromatic compounds, which include nitrotoluenes such as mononitrotoluene and dinitrotoluene, are generally commercially produced by the nitration of the corresponding aromatic hydrocarbon. The nitration reaction is typically carried out by reacting the aromatic hydrocarbon with nitric acid in the presence of sulfuric acid, or with nitric acid alone. Examples of such nitration reactions are illustrated in the following patents:
U.S. Pat. No. 2,362,743 describes the preparation of dinitrotoluene by the nitration of toluene with nitric acid of about a 70% concentration to produce mononitrotoluene, followed by further nitration with nitric acid of about 98% concentration to produce a crude dinitrotoluene. This patent states that the use of the concentrated nitric acid is an improvement over the use of sulfuric acid alone and mixtures of sulfuric and nitric acids as nitrating agents. The crude dinitrotoluene produced by the process of the patent is stated to be subjected to several steps of washing, neutralizing, alcohol treatment, and centrifuging to obtain the 2,4-dinitrotoluene.
U.S Pat. No. 4,804,792 describes the nitration of benzene and toluene by contacting these with concentrated nitric acid in the presence of a molten nitrate salt. The patent states that the molten salt serves as a temperature regulator for the reaction and as an isothermal medium for the reactants. A preferred method of contacting the reactants in the presence of the molten salt is stated to be by bubbling the reactants into a bath of the molten salt by means of a carrier gas such as nitrogen. The desired product is stated to be separated out by a variety of well known procedures.
U.S. Pat. No. 4,918,250 describes a process for nitrating toluene to DNT and phase separation of the product using an inorganic salt as a phase separation agent. In this patent, DNT is produced in a two-step liquid phase nitration reaction between nitric acid and toluene in the absence of sulfuric acid and solvent. In the process of the patent, the inorganic salt is incorporated into the mixture of DNT and unreacted nitric acid in an amount sufficient to cause phase separation of the mixture in order to facilitate isolation of the DNT from the unreacted nitric acid in the product mixture (column 2, lines 27 to 33). After phase separation, the patent states that washing with water and a basic solution produces a purified DNT (column 3, lines 13 to 15).
The processes discussed, as well as commercial processes nitrate the aromatic compound with a strong acid to produce a nitro substituted aromatic compound. A separation step is required to separate the acid from the product. The crude product is generally washed with water and dilute alkaline solutions. The spent wash solution is then treated before discharge or recycle to the nitration process. If process by-products are present, their separation is more difficult by the bulk of the water. The large quantity of water used requires excessive energy to recover the by-products by distillation. Further, the product nitro substituted aromatic compound is still generally contaminated with the acid.
To be commercially usable for many applications the nitro substituted aromatic compound must have an acid content of less than 3%. For example, DNT is useful as an intermediate for producing toluene diisocyanate (TDI). This requires that the contaminant acid must be essentially completely removed. The techniques described above which utilize large amounts of water and an alkaline solution do not completely separate all of the acid from the product.
Accordingly, alternate separation means would be desirable to remove essentially all of the acid contained in the product nitro substituted aromatic compound.